The mechanisms by which a T cell response to a foreign (allogeneic or xenogeneic) protein or cell or organ is mounted are fairly well understood. Antigen presenting cells (APCs) are attracted to areas of inflammation or damage (that may be induced by surgical transplantation). The repertoire of T cells in the periphery is constantly surveying tissues for evidence of pathogens or the presence of foreign (allo- or xenogeneic) tissue. Once any of these warning signals are recognised, the APCs engulf the protein, digest it and present it to the host's immune system.
The immune system is well equipped to rapidly identify foreign, diseased or inflamed tissue and rapidly destroys it. This has always been a major barrier to tissue, organ and cell transplantation as well as gene therapy. Major problems are generally associated with chronic inmmunosuppression, encapsulation or immunoisolation. The unwanted side effects of chronic immunosuppression include increased susceptibility to opportunistic infection and tumour formation.
In particular, acute renal allograf rejection is mediated by both alloantigen-dependent and -independent factors and is characterised by a mononuclear cell infiltrate consisting mainly of T lymphocytes, monocyte/macrophages and occasional eosinophils (Gröne H. J., 1996, Valente J. F. et al., 1998, Bishop G. A. et al., 1986). The recruitment of these leukocytes from the peripheral circulation into the transplanted organ involves a complex interplay between a series of molecules expressed on the leukocyte and endothelial surface (Butcher E. C., 1991, Butcher E. C. et al., 1996, Springer T. A., 1994).
The desire for long-term acceptance of grafted tissue in the absence of continuous immunosuppression is a long-standing goal in human medicine.
Chemokines, a large superfamily of structurally related cytokines, have been shown to selectively promote the rapid adhesion, chemotaxis and activation of specific leukocyte effector subpopulations (Springer T. A., 1994, Nelson P. J. et al., 1998, Luster A. D., 1998. Schlöndorff D. et al., 1997).
Chemokines are characterised by a series of shared structural elements including the conserved cysteine residues used to define the C, C—C, C—X—C and C—X3—C chemokine subgroups (where X represents an intervening amino acid residue between the first two amino terminal proximal cysteines). All of the various biological actions of chemokines appear to be directed through their interaction with a large family of seven-transmembrane spanning, C-protein coupled receptors (Nelson P. J. et al., 1998, Luster A. D., 1998, Schlöndorff D. et al., 1997). The cell type specific expression of these receptors appears to control a significant degree, the leukocyte specificity of chemokine action (Nelson P. J. et al, 1998, Luster A. D., 1998, Schlöndorff D. et al., 1997).
The chemokine RANTES (regulated upon activation, normal T-cell expressed and secreted), a member of the C—C chemokine subfamily, is a ligand for a number of chemokine receptors including CCR1, CCR3, CCR5, CCR9 and DARC (Duffy Antigen Receptor for Chemokines) in humans (Nelson P. J. et al., 1998, Luster A. D., 1998, Schlöndorff D. et al., 1997, Nibbs R. J. et al., 1997). RANTES is a potent chemoattractant for T cells, monocytes, natural killer cells, basophils and eosinophils (Nelson P. J. et al., 1998).
Chemokines such as RANTES, are thought to play pivotal roles in the cellular infiltrates that underlie various disease processes. For example, RANTES is expressed in vivo in diseases characterised by a mononuclear cell infiltrate including, delayed-type hypersensitivity, necrotizing glomerulonephritis, inflammatory lung disease and renal allograft rejection (Schlöndorff D. et al., 1997, Nelson P. J. et al., 1998, Devergne O. et al., 1994, Luckas N. W. et al., 1996, Lloyd C. M. et al., 1997, Pattison J. et al., 1994, Wiedermann C. J. et al., 1993). In studies of human kidneys undergoing acute cellular rejection, RANTES protein was found localised to mononuclear infiltrating cells, renal tubular epithelial cells and tile endothelium of peritubular capillaries (Pattison J. et al., 1994, Wiedermann C. J. et al., 1993). Since acute cellular rejection is characterised by an intravascular mad interstitial cellular infiltrate consisting of monocyte/macrophages, T lymphocytes and occasional eosinophils, RANTES is potentially a key player in the pathogenesis of acute rejection (Schlöndorff D. et al., 1997, Nelson P. J. et al., 1998, Pattison J. et al., 1994, Wiedermann C. J. et al., 1993).
Based upon these observations a model for the role of RANTES in renal allograft rejection was proposed (Nelson P. J. et al., 1998, Pattison J. et al., 1994, Wiedermann C. J. et al., 1993). Early in rejection, the microvascular endothelium becomes inflamed, platelets degranulate, releasing RANTES protein that binds to the endothelial surface. The inflamed renal tubules and endothelial cells produce additional chemokines including RANTES. The accumulated surface bound chemokines then provide directional signals to circulating leukocytes as they roll across the endothelial surface (Butcher E. C., 1991, Butcher E. C. et al., 1996, Springer T. A., 1994, Nelson P. J. et al., 1998, Pattison J. et al., 1994, Wiedermann C. J. et al., 1993). Leukocytes recognise the surface bound protein, upregulate integrins, and firmly adhere to the endothelial surface, undergo diapedesis and extravasation. As the leukocytes become activated, they produce additional cytokines and chemokines thus amplifying and propagating the inflammatory response (Nelson P. J. et al., 1998, Pattison J. et al., 1994, Wiedermann C. J. et al., 1993).
Modification of the amino terminus of the RANTES protein can dramatically alter its properties (Proudfoof A. E. et al., 1996, Gong J. H. et al., 1996, Simmons G. et al., 1997). The addition of a single methionine residue changes the agonist protein into a RANTES receptor antagonist with nanomolar potency (Proudfoof A. E. et al., 1996). This antagonist, Met-RANTES, is bioactive in mouse and rat (Proudfoot unpublished), and has been shown to suppress inflammation in murine models of allergic skin and rheumatoid arthritis and to partially inhibit in necrotizing glomerulonephritis (Teixeira M. M et al., 1997, Plater-Zyberk C. et al., 1997, Lloyd C. M et al., 1997).
Cyclosporins represent a group of nonpolar cyclic oligopeptides, having imnnunosuppressant activity, produced by the fungus Tolypocladium inflatum Gams and other fungi imperfecti. The major component, cyclosporin A, has been identified along with several other minor metabolites, cyclosporins B through N. A number of synthetic analogues have also been prepared. Cyclosporin A is a commercially available drug, which has attained widespread clinical application as immunosuppressant in organ transplantation procedures.
The main problem with cyclosporin A has been its nephrotoxicity (Martindale, 1996), characterised by fluid retention, increased serum creatinine and urea concentrations, a fall in glomerular filtration rate, and decreased sodium and potassium excretion. In particular, in renal graft recipients may be difficult to distinguish nephrotoxicity from graft rejection.